Many systems for production of three-dimensional modeling by photohardening have been proposed. European Patent Application No. 250,121 filed by Scitex Corp. Ltd. on Jun. 6, 1987, provides a good summary of documents pertinent to this art area, including various approaches attributed to Hull, Kodama, and Herbert. Additional background is described in U.S. Pat. No. 4,752,498 issued to Fudim on Jun. 21, 1988.
These approaches relate to the formation of solid sectors of three-dimensional objects in steps by sequential irradiation of areas or volumes sought to be solidified. Various masking techniques are described as well as the use of direct laser writing, i.e., exposing a photohardenable polymer with a laser beam according to a desired pattern and building a three-dimensional model layer by layer.
However, all these approaches fail to identify practical ways of utilizing the advantages of vector scanning combined with means to maintain constant exposure and attain substantially constant final thickness of all hardened portions on each layer throughout the body of the rigid three dimensional object. Furthermore, they fail to recognize very important interrelations within specific ranges of operation, which govern the process and the apparatus parameters in order to render them practical and useful. Such ranges are those of constant exposure levels dependent on the photohardening response of the material, those of minimum distance traveled by the beam at maximum acceleration dependent on the resolution and depth of photohardening, as well as those of maximum beam intensity depend on the photospeed of the photohardenable composition.
The Scitex patent, for example, suggests the use of photomasks or raster scanning for achieving uniform exposure, but does not suggest a solution for keeping the exposure constant in the case of vector scanning. The use of photomasks renders such techniques excessively time consuming and expensive. Raster scanning is also undesirable compared to vector scanning for a number of reasons, including:
necessity to scan the whole field even if the object to be produced is only a very small part of the total volume, PA0 considerably increased amount of data to be stored in most cases, PA0 overall more difficult manipulation of the stored data, and PA0 the necessity to convert CAD-based vector data to raster data. PA0 an ethylenically unsaturated monomer, a photoinitiator, and an additive mixed therein, the additive being soluble in the composition but separating into a separate phase upon photohardening the composition, the additive occupying a first volume in the absence of all other components of the composition, the photohardenable composition occupying a second volume in the absence of the additive mixed therein, and the photohardenable composition containing the additive occupying a third volume, PA0 wherein the third volume is smaller than the sum of the first volume and the second volume. PA0 (a) forming a layer of a photohardenable liquid; PA0 (b) photohardening at least a portion of the layer of photohardenable liquid by exposure to actinic radiation; PA0 (c) introducing a new layer of photohardenable liquid onto the layer previously exposed to actinic radiation; PA0 (d) photohardening at least a portion of the new liquid layer by exposure to actinic radiation, with the requirement that the photohardenable composition comprises an ethylenically unsaturated monomer, a photoinitiator, and an additive mixed therein, the additive being soluble in the composition but separating into a separate phase upon photohardening the composition, the additive occupying a first volume in the absence of all other components of the composition, the photohardenable composition occupying a second volume in the absence of the additive mixed therein, and the photohardenable composition containing the additive occupying a third volume,
On the other hand, in the case of vector scanning only the areas corresponding to the shape of the rigid object have to be scanned, the amount of data to be stored is smaller the data can be manipulated more easily, and "more than 90% of the CAD based machines generate and utilize vector data" (Lasers & Optronics, January 1989, Vol. 8, No. 1, pg. 56). The main reason why laser vector scanning has not been utilized extensively so far is the fact that, despite its advantages, it introduces problems related to the inertia of the optical members, such as mirrors, of the available deflection systems for the currently most convenient actinic radiation sources, such as lasers. Since these systems are electromechanical in nature, there is a finite acceleration involved in reaching any beam velocity. This unavoidable non-uniformity in velocity results in unacceptable thickness variations. Especially in the case of portions of layers having no immediate previous levels of exposure at the high intensity it becomes necessary to use high beam velocities, and therefore, longer acceleration times, which in turn result in thickness non-uniformity. The use of low intensity lasers does not provide a good solution since it makes production of a solid object excessively time consuming. In addition, the usefulness of vector scanning is further minimized unless at least the aforementioned depth and exposure level relationships are observed as evidenced under the Detailed Description of this invention.
No special attention has been paid so far to the composition itself by related art in the field of solid imaging, except in very general terms.
Thus, the compositions usually employed, present a number of different problems, one of which is lack of good dimensional control, especially during the step of photohardening. Photohardening, is usually followed by shrinkage which in turn gives rise to internal stresses. Shrinkage itself results in parts which are smaller than intended to be, and internal stresses may result in warpage and cracking, as well as strength reduction.
In other fields of art, reduction of shrinkage has been proposed for different purposes.
U.S. Pat. No. 4,590,242 (Horn et al.) describes low shrinkage nylon molding compositions comprising lactam monomer and a polymer which separates out in the course of photohardening. The polymer is composed of one or more blocks which are compatible with polylactam and one or more blocks which are incompatible therewith.
U.S. Pat. No. 4,245,068 (Brewbaker et al.) describes thermosettable compositions comprising an unsaturated polyester and a vinyl monomer capable of crosslinking the polyester, where the shrinkage is reduced during cure by the addition to the composition of an alkylene oxide polymer which is soluble in the thermosettable composition but which is substantially soluble in the cured thermoset composition.
U.S. Pat. No. 4,151,219 (Brewbaker et al.) describes similar compositions as U.S. Pat. No. 4,245,068.
U.S. Pat. No. 3,701,748 (Kroekel) describes a composition curable under heat and pressure for molding, containing a thermoplastic polymer which is soluble in the composition, but yields an optically heterogeneous cured composition.
British Patent 1,276,198 describes similar compositions as U.S. Pat. No. 3,701,748.
U.S. Pat. Nos. 4,078,229, 4,288,861, and 4,446,080 (Swainson et al.) describe holographic techniques utilizing two or more beams for multiple proton absorption for production of physical or refractive index inhomogeneities at the intersection of the beams.
It is therefore an object of the present invention to provide compositions and a method for producing parts by solid imaging, characterized by improved dimensional control, reduced warpage potential, and greater strength.